Various types of linear motion elements are known, which convert external force into linear motion. Fluid-operated devices which use compressed hydraulic fluid, compressed air, or the like, usually utilize a tube, in the form of a cylinder, within which a piston is longitudinally movable. To transfer differential force applied to the fluid to linear motion with minimum space, the cylinder is usually formed with a longitudinal slit through which a force transfer element extends, coupled to a piston, or other cylindrical element which can operate within the cylinder. The slit is sealed at both sides of the force transfer element to prevent escape of the working fluid from the cylinder.
Such force-to-linear movement conversion elements require a pressure source, for hydraulic or pneumatic media, which is not always available. Precise positioning of the force transfer element with respect to a reference mark is difficult to obtain, and additional auxiliary apparatus is frequently needed, such as stops, engagement shoulders, elements or the like, if a particular predetermined position of the force transfer element with respect to the housing, for example of the cylinder, is to be obtained. Such particular positions, however, are frequently needed if the unit is to be used as a positioning element, for example to locate a particular machine tool element or a workpiece at a specific point.
Spindle drives are suitable for many applications. They are independent of pressure sources, and permit good reproducibility of positioning, with respect to input rotary power.